Milling plant and method for milling raw materials

ABSTRACT

In a mill drying plant with a roller grinding mill and preferably in a cement production plant operated in compound with a clinker burning process and having a kiln fan and a fan downstream of a filter, a mill fan is connected upstream of the roller grinding mill as a further fan. This makes it possible to overcome the resistance of the roller grinding mill blade ring and to force the pressure zero point into the roller grinding mill, so that there is a low vacuum above the blade ring in the milling and classifying chamber and in the filter.

FIELD OF THE INVENTION

[0001] The invention relates to a milling plant having a roller grindingmill, a classifier and an exhaust gas fan, which follows a filter,particularly a plant for the production of cement, and to a method forcement production in a compound system.

BACKGROUND OF THE INVENTION

[0002] In the production of cement raw material mill drying plants areoperated in combination with a clinker burning process, in order tosupply the exhaust gases formed in a cement rotary kiln, following heatexchangers, to a further heat utilization.

[0003] A known milling system can be gathered from a circuit diagramaccording to FIG. 3. Exhaust gases 3 from a calcining process aresupplied by a kiln fan 5 into an exhaust gas pipe 6, via a shutoffdevice 6 a to a roller grinding mill 2. The term kiln fan 5 is used hereto define the fan feeding a gas flow to a mill. In combined circuits ofcement rotary kilns and heat exchangers with an air-swept rollergrinding mill, the kiln or kiln/heat exchanger-fan supplies the kiln gasflow and heat flow to a roller grinding mill, e.g. an air swept rollergrinding mill. The term mill fan is used to define the fan, whichdelivers the gas flow necessary for the mill and which as fan 7 ispositioned in the flow direction below the filter 8 functioning as adust separator. Said fan 7 supplies the exhaust gas 3 with fines or meal34 produced in the roller grinding mill 2, by means of a dust-exhaustgas pipe 9, which contains a shutoff device 9 a, to the filter 8. Thefines 34 separated in the filter 8 are delivered by means of not shownconveying systems into silos. The dust-freed exhaust gas 3 is suppliedwith the aid of the fan 7 following the filter 8 to a not shown chimneyflue and ejected.

[0004] In a compound operation, in which the milling plant is operatedon line with a not shown cement rotary kiln, a shutoff device 15 a in abypass line 15 remains closed. Unusable thermal energy is dissipated ina cooling tower 22, which generally follows the kiln fan 5, or in theroller grinding mill 2, e.g. by water injection.

[0005] Individual components of a raw material mixture 33 are suppliedfrom bunkers 31 by means of weighting belts 30 to a feed belt 32 and fedto the roller grinding mill 2. The feed device can be constituted by aflap lock 25, which acts as an air excluder. Coarse material 26separated in the roller grinding mill 2 is at least partly admixed withthe raw material mixture 33 by means of a conveying machinery 24 and issupplied again to the roller grinding mill 2.

[0006] In combined operation the milling plant according to FIG. 3 isoperated in such a way that the pressure-zero point, i.e. the point ofatmospheric pressure, is located shortly upstream of the entry of theexhaust gases into the roller grinding mill 2, so that said grindingroller mill 2, an integrated classifier 13 and the filter 8 operateunder a relatively high sub-atmospheric pressure and correspondingsealing mechanisms and stable structures are required, which will bediscussed hereinafter.

[0007] In direct operation the milling plant according to FIG. 3 is outof action and only the not shown cement rotary kiln is operated. Incombined operation the dust produced by the mill 2 and the residual dustfrom the heat exchanger are separated in the filter 8, so that the term“filter plant” is used. However, in direct operation only the residualdust from the exhaust gas 3 from the not shown heat exchanger isseparated. The shutoff device 6 a in the exhaust gas pipe 6 and theshutoff device 9 a in the dust-exhaust gas pipe 9 are then closed andthe exhaust gas flow 3 from the calcining process, following the coolingtower 22, is passed directly via the bypass line 15 into the filter 8,where it is dedusted and fed by means of the following fan 7 and a notshown chimney flue into the atmosphere. In direct operation the fan 7serves as an exhaust gas fan for the heat exchanger.

[0008] For the production of cement raw material independently of thecalcining process, e.g. when the cement rotary kiln is out of action oron recommissioning, as no exhaust gas is available, fresh air 4 issupplied by means of a control flap 4 a and a hot gas generator 37 tothe roller grinding mill 2. The shutoff devices 6 a and 15 a in theexhaust gas pipe 6 and bypass line 15 are then closed, whereas theshutoff device 9 a in the dust exhaust gas pipe 9 between rollergrinding mill 2 and filter 8 is opened.

[0009]FIG. 4 shows a circuit diagram of a further raw material milldrying system, which is also known as the “three-blower version”. Foridentical features the same reference numerals as in FIG. 3 are used. Afirst blower, which corresponds to the kiln fan 5 of FIG. 3, but whichis not shown, is located in the flow direction upstream of a coolingtower 22 and feeds the exhaust gases 3 a from the calcining process to aroller grinding mill 2. A second blower 28, which acts as a mill fan, islocated downstream of a multiple-unit cyclone 29 and feeds a partial gasflow, via a return line 16 back into the roller grinding mill 2. Theremaining exhaust gas part from the multiple-unit cyclone 29 is suppliedby means of a control and shutoff device 14 to a filter 8. The filter 8is followed by a third blower 38 as exhaust gas fan and which feeds theremainining exhaust gases 3 into a not shown chimney flue as filterexhaust gas. The fines 34, 35 separated in the multiple-unit cylone 29and in filter 8 are supplied by corresponding conveyer mechanisms 36 toa not shown silo. Reference is made to the remarks concerning FIG. 3 inconnection with the devices for the production and supply of a rawmaterial mixture 33 to the roller grinding mill 2.

[0010] The mill drying plant according to FIG. 4 can operateindependently of the gas and thermal balance of the compound system. Bymeans of a bypass line 15 excess exhaust gas and heat flows from thekiln and calcining process can be bypassed the roller grinding mill 2with classifier 13 and multiple-unit cyclone 29 and can be dedustedtogether with the exhaust gases from the roller grinding mill 2 in thefollowing filter plant 8.

[0011]FIG. 6 shows an exemplified pressure curve of the combined circuitvariant of the plant of FIG. 4. The essential devices with thecorresponding association are shown above the pressure curve and giventhe reference numerals of FIG. 4. FIG. 6 shows that the filter 8operates in a relatively low sub-atmospheric pressure range andtherefore only has to be protected against air infiltrations byrelatively limited expenditure. However, the roller grinding mill 2,which is operated with a vacuum of about −50 to −80 mbar, must bevirtually “hermetically” sealed to avoid air infiltrations.

[0012] Disadvantages also result from the multiple-unit cyclone 29,which is associated with relatively high construction and spaceconsumption costs and is subject to wear, so that there are also highmaintenance costs.

[0013] A high degree of separation in the multiple-unit cyclone 29 alsorequires relatively high energy costs and a further disadvantage of theknown plant is the splitting up of the end product into coarser fines 35from the cyclones and finer fines 34 from the filter 9 (FIG. 4).

[0014] The flexibility of the milling plant is limited, because theseparation level of the multiple-unit cyclone 29 is coupled with theload state (=gas flow) of the mill. The cyclone separation level dropsin the case of a partial load, so that there is a rise in the residualdust content in the line 39 following the multiple-unit cyclone 29 tothe mill fan 28, which leads to wear phenomena.

[0015] Although admittedly the mill drying plant according to FIG. 3 hasa simpler pipe layout, lower energy costs and relatively low capitalcosts due to a compact plant construction. As can be seen from theexemplified pressure curve of the compound circuit variant of the plantaccording to FIG. 3 shown in FIG. 5, the filter 8 is incorporated intothe vacuum range of the roller grinding mill 2, which is approximately−70 to −90 mbar and must therefore be correspondingly designed from theconstruction and safety standpoints. Filter housings are of considerablesize in industrial milling plants. The filter housing must be designedfor operational vacuums of almost −100 mbar and for a cold air start to−140 mbar for safety reasons. Considerable constructional and productionmeasures are necessary to achieve the necessary rigidity and prevent acollapse. Even as a result of small cracks in the housing, the followingmill fan 7 sucks secondary air into the plant, which acts as a loss onthe mill exhaust gas and leads to functional problems.

[0016] Pressure fluctuations, which are unavoidable in the enlargedvacuum range, lead to high mechanical loading of the filter housing andto a high susceptibility to cracking and therefore air infiltrations.These directly influence the capacity of the roller grinding mill. Inaddition, the not “hermetically” sealed points of the roller grindingmill and the classifier, together with corresponding areas in theoverall system are potential sources for air infiltrations. The damagepoints can also not be located from the outside due to a closedinsulation.

[0017] Another disadvantage is that the exhaust gases 3 must be drawnout of the heat exchanger unit of the calcining process during compoundoperation through the roller grinding mill 2 and a bypassing of theexhaust gases is impossible (cf. also FIG. 3).

[0018] The gas temperature in the dedusting filter 8 cannot be keptindependent of the mill exhaust gas temperature. This dependence canalone and together with air infiltrations lead to a local dropping belowthe dew point and therefore to corrosion in the filter and pipe area.During mill operation no other gases than those from the roller grindingmill 2 can be dedusted.

SUMMARY OF THE INVENTION

[0019] The object of the invention is to provide a milling plant and amethod for the milling of raw materials, particularly a cementproduction plant and a method, which with a particularly low capital andmaintenance costs ensure a variable method performance and an extremelyefficient milling process and in particular an efficient cementproduction.

[0020] From the apparatus standpoint, this object is achieved by amilling plant, e.g. by a plant for the production of cement with aroller grinding mill, a cement rotary kiln with a heat exchanger unit, akiln fan and an exhaust gas fan downstream of a filter, in which a millfan is positioned upstream of the roller grinding mill and overcomes thehigh resistance of the roller grinding mill blade ring and forces anexhaust gas or also air flow through the roller grinding mill blade ringnecessary for a mill drying process and, in cooperation with the exhaustgas fan, permits an advantageous pressure curve upstream, within anddownstream of the roller grinding mill and extending to the filter.

[0021] From the method standpoint, according to the invention theprepositive mill fan positioned upstream of the mill forces the pressurezero point, i.e. the atmospheric pressure into the roller grinding milland operationally reliably held in the area of the blade ring andmilling pan, particularly in an upper area of the blade ring andextending to an upper edge of the milling pan. In the milling chamber ofthe roller grinding mill above the milling pan and blade ring, includingat the passages for the levers and/or spring rods, in the classifier andin the filter, a minimun sub-atmospheric pressure is set by means of theexhaust gas fan, whereas in the flow direction upstream of the bladering, particularly in the lower part of the mill, an overpressure isproduced by the mill fan.

[0022] Since, according to the invention, lower sub-atmospheric pressurelevels can be set than in the known mill drying plants, it isadvantageously achieved that it is possible to move away from the known,stable costly constructions of the filter or cyclones and furthermilling plant equipment and a considerable percentage of the airinfiltration sources is eliminated.

[0023] The disposition of a mill fan upstream of a roller grinding millis known per se from coal milling plants. However, in that case the millfan is the sole fan in the coal milling system. In addition, the millingchamber, classifying chamber and e.g. the following burner lines andcoal dust/air mixture burners are under an overpressure.

[0024] According to the invention, the upstream mill fan is a compoundcement raw material milling plant with mill drying process serves todisplace the pressure level in such a way that in cooperation with theexhaust gas fan in the mill interior there is a much lowersub-atmospheric pressure extending to ambient pressure. The totalcapacity of the fans of the mill drying plant, i.e. the mill fan andexhaust gas fan downstream of the filter system, is distributed over thefans and is lower due to the reduced air infiltrations. The lower partof the mill is under an overpressure. The filter can be operated with amuch lower vacuum than in the prior art in accordance with the circuitof FIG. 3.

[0025] Thus, the construction of the filter housing for absorbing thesurface pressure can be significantly simplified in avantageous manner.

[0026] The gas flow through the roller grinding mill is controlled bythe upstream mill fan. It is advantageous that the control of a bypassexhaust gas flow can be brought about without difficulty by the exhaustgas fan due to the smaller pressure gradient to the filter. Due to thereduced pressure level upstream of the filter, it is also possible toconnect dedusting devices to the line between the roller grinding milland the filter.

[0027] Essential advantages of the plant and method according to theinvention with a clearly defined pressure curve in the mill system arethe omission of complicated seals against air infiltrations in theroller grinding mill, e.g. in the material feed area, energy savings dueto the air infiltrations which no longer occurs, reduced capital andoperating costs, together with flexible method performance incombination and the possibility of being able to dedust other exhaustgases in the same filter plant. A considerable cost saving moreparticularly results from the lighter construction of the filter housingand the omissions of cyclones.

[0028] Besides the economies in capital expenditure, maintenance costsand costs for increased energy consumption are avoided.

[0029] The invention cannot only be used with advantage for cement rawmaterial milling plants. It falls within the scope of the invention tooperate a milling plant without any upstream process, e.g. a so-calledcentral milling plant, using an upstream mill fan, in order to displacethe pressure level from the sub-atmospheric pressure to the overpressurerange into the milling chamber of the roller grinding mill and toproduce a vacuum by an exhaust gas fan following the filter. Consideredin the flow direction, the lower milling chamber up to the suctionconnection of the exhaust gas fan is under a lower vacuum than inconventional milling plants.

[0030] The advantages are as in the case of a cement raw materialmilling plant, namely considerable energy and capital cost savings, theavoiding of air infiltrations, maintenance costs, and a variable methodperformance.

[0031] An inventive milling plant, in which an upstream mill fancooperates with an exhaust gas fan, can be used for milling the mostvaried raw materials such as clinker, metallurgical sand, coal and allother mineral and rock types.

[0032] In the so-called filter plants, the use of a fan upstream of themill means that there is a further fan besides the exhaust gas fan. Inthe hitherto conventional filter plants the fan, positioned behind thefilter, acts both as a mill fan and as an exhaust gas fan. Theabove-described advantages of the modified pressure level (MPL) processperformable with an upstream mill fan make the additional costs of a fanappear extremely small.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention is described hereinafter relative to the attacheddrawings.

[0034]FIG. 1 is a plant diagram of a milling plant according to theinvention;

[0035]FIG. 2 is a diagrammatic representation of an exemplified pressurecurve of the inventive compound circuit variant of FIG. 1;

[0036]FIGS. 3 and 4 is a plant and circuit diagrams of mill dryingplants according to the prior art;

[0037]FIGS. 5 and 6 are pressure curves of the plant and circuitdiagrams of FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 shows a circuit diagram of a plant for cement productionwith a calcining process and a mill drying process.

[0039] Exhaust gases 3 from a cement rotary kiln 40 are supplied bymeans of a precalciner 41, a heat exchanger unit 42 and a kiln fan 5 aswell as a cooling tower 22 in an exhaust gas pipe 6 to a mill dryingprocess with a roller grinding mill 2 and a filter 8. A bag orelectrostatic filter can be used as filter 8.

[0040] Upstream of the roller grinding mill 2 with a milling pan 12 anda blade ring 11 or nozzle ring in a milling chamber and an integratedclassifier 13 in a classifying chamber is provided a mill fan 10 or millblower, with which it is possible to raise the pressure level in theregions of the roller grinding mill 2 and filter 8.

[0041] The overall capacity of the milling plant is distributed over theupstream mill fan 10 and exhaust gas fan 7 and is lower instead ofhigher, due to the reduced air infiltrations.

[0042] The upstream mill fan 10 forces the exhaust gas flow necessaryfor the mill drying process through the blade ring 11 and consequentlyprevents the latter acting as a resistance for the exhaust gas fan 7 anda high vacuum gradient to the atmosphere in the upper part of the milland the following plant parts, particularly in filter 8 is built up. Thehigher said vacuum rises, the more secondary air can be sucked into thesysstem and the less exhaust gas is drawn through the blade ring 11.

[0043] Through the pressure level displacement the secondary airpercentage in the system is significantly reduced. Apart from the energyand method advantages, the housing of the filter 8 can be made lighterand therefore less expensively.

[0044] The exhaust gas pipe 6 contains a control and shutoff device 46,e.g. a throttle valve, which permits a bypassing of the exhaust gases 3via a bypass line 15 and a dedusting in filter 8 and not only when theroller grinding mill 2 is not operating.

[0045] In front of the upstream mill fan 10 is located a gas flowmeasuring device 20, whose measured values are used for controlling themill fan 10.

[0046] The devices and plant parts for producing a raw material mixtureand supply and feed devices for the roller grinding mill 2 are not shownin FIG. 1 and can be constructed as in the plants of FIGS. 3 and 4.

[0047] A dust-exhaust gas pipe 9 leads from the roller mill 2 to thefilter 8. The fines 34 separated in the filter 8 are conveyed to silosby not shown conveying mechanisms. The dedusted exhaust gas 3 can atleast partly be returned by means of a return line 16 to the mill fan 10of the roller grinding mill 2. If a controllable shutoff device 47 inthe return line 16 is closed, the dedusted exhaust gases 3 are passedvia a chimney flue into the atmosphere. FIG. 1 shows possibilities forcontrolling the shutoff and control device 46 in the exhaust gas pipe 6,a control and shutoff device 48 in bypass line 15, the control andshutoff device 47 in return line 16 and in the dust-exhaust gas pipe 9.

[0048] The clearly defined pressure curve, which can be obtained with anupstream mill fan 10 in cooperation with an exhaust gas fan 7 in themill drying plant according to FIG. 1 is shown in exemplified manner inFIG. 2.

[0049] Above the pressure curve are shown the essential plant parts,namely a kiln fan 5, the upstream mill fan 10, the roller grinding mill2, the filter 8 and the exhaust gas fan 7. FIG. 2 illustrates themodified pressure level upstream, within and downstream of the rollergrinding mill 2 and in the filter 8. A comparison with the exemplifiedpressure curves of the known plant systems, shown in FIGS. 5 and 6,illustrates that in the known milling systems directly prior to theentry of the gas flow into the roller grinding mill 2 there is a vacuumof approximatelx −5 to −7 mbar and in the roller grinding mill 2 avacuum of −50 to −70 mbar. The vacuum in the electrostatic filter in theknown plant according to FIGS. 3 and 5 of around −90 mbar isparticularly disadvantageous.

[0050]FIG. 2 illustrates that with the upstream mill fan 10 directlyupstream of the roller grinding mill 2 or in the lower part of the milland upstream of the blade ring 11, it is possible to set anoverpressure, e.g. of approximately 40 mbar. Operationally the pressurezero point is in an upper area of the blade ring 11 and extending to anupper edge of the milling pan 12. In the milling and classifying chamberin the example of FIG. 2 there is a vacuum of approximately −3 to −25mbar and in the filter 8 only a vacuum of approximately -−40 mbar.

[0051] The modified pressure level (MPL) method according to theinvention is consequently characterized by a displaced pressure levelupstream, within and downstream of the roller grinding mill 2, which isachieved with the upstream mill fan 10 in cooperation with the exhaustgas fan 7 and which leads to extraordinary savings of capital, energyand maintenance costs and to a particularly efficient crushing of rawmaterials and in particular to efficient cement production.

1. Milling plant with a roller grinding mill, particularly an air-sweptroller mill, having a milling pan, a blade ring and a classifier andhaving a fan, located downstream of a filter and supplies asub-atmospheric pressure to the filter, roller grinding mill and adust-gas pipe between the filter and roller grinding mill, wherein amill fan is positioned upstream of the roller grinding mill and forcesthe necessary gas flow into the roller grinding mill and apredeterminable pressure level can be set upstream, within anddownstream of the roller grinding mill by the mill pan and exhaust gasfan.
 2. Milling plant according to claim 1, wherein the mill fan isupstream of the roller grinding mill and the pressure zero point can belocated in the area of the blade ring and milling pan.
 3. Plantaccording to claim 2, wherein the pressure zero point or a minimalsub-atmospheric pressure can be set in an upper area of the blade ringextending to an upper edge of the milling pan.
 4. Plant according toclaim 3, wherein with the upstream mill fan upstream and within theroller grinding mill below the blade ring can be set an overpressure andabove the milling pan and in the classifier which is integrated into theroller grinding mill can be set a sub-atmospheric pressure.
 5. Plantaccording to claim 4, wherein a sub-atmospheric pressure can be set inthe filter which has a relatively lightweight housing construction. 6.Plant according to claim 5, wherein the roller grinding mill hassimplified seals in the area of the raw material feed and passages ofmoving parts through the mill housing.
 7. Plant according to claim 6,wherein between the roller grinding mill and filter dedusting devicescan be connected to the dust-exhaust gas pipe.
 8. Plant according toclaim 7, wherein for the production of cement, the roller grinding millis in a compound circuit with a cement rotary kiln, a heat exchangerunit and a kiln fan, which is positioned downstream of a heat exchangerunit and supplies the exhaust gases from the cement rotary kiln to anexhaust gas pipe and that with the mill fan positioned upstream of theroller grinding mill a necessary exhaust gas flow can be supplied to theroller grinding mill for setting a predeterminable pressure levelupstream, within and downstream thereof.
 9. Plant according to claim 8,wherein there is a bypass line for supplying exhaust gases from thecement rotary kiln to the filter and that the bypass line branches offin the flow direction from the exhaust gas pipe upstream of a controland shutoff device.
 10. Plant according to claim 9, wherein the exhaustgas flow to the filter can be conditioned largely independently of thegas flow through the roller grinding mill with respect to the volumeflow and gas temperature.
 11. Plant according to claim 10, wherein forthe return of controllable exhaust gas flows to the roller grindingmill, a return line is positioned downstream of the filter.
 12. Plantaccording to claim 11, wherein control and shutoff devices are locatedin the exhaust gas pipe, as well as in the bypass line and return line.13. Plant according to claim 8, wherein a gas flow measuring device islocated in the supply line to the roller grinding mill upstream of themill fan.
 14. Method for production of cement in a compound system, inwhich a raw material mixture undergoes mill drying in a roller grindingmill, accompanied by the supply of exhaust gases from a calciningprocess, followed by classification and supply as a dust-exhaust gasmixture to a filter for dust separation from the exhaust gas and theexhaust gases are passed with the aid of a kiln fan following calciningand preheating of raw material and with an exhaust gas fan following afilter through the roller grinding mill and the filter in a compoundcircuit, particularly in a plant according to claim 8, wherein acontrollable exhaust gas quantity is forced by a mill fan locatedupstream of the roller grinding mill into the latter and through theblade ring thereof, wherein the pressure zero point is placed in theplane of the blade ring and the milling pan of the roller grinding milland wherein above the blade ring and extending into the filter a lowsub-atmospheric pressure is set.
 15. Method according to claim 14,wherein an overpressure is set upstream and below the blade ring of theroller grinding mill with the aid of the upstream mill fan and incooperation with the exhaust fan.
 16. Method according to claim 15,wherein an overpressure of 40 to 50 mbar is set.
 17. Method according toclaim 15, wherein in the vicinity of the blade ring and the milling panis set a minimum sub-atmospheric pressure and in the vicinity of theclassifier, dust-exhaust gas pipe and filter is set a vacuum.
 18. Methodaccording to claim 17, wherein in the vicinity of the blade ring andmilling pan is set a vacuum of approximately −3 to −5 mbar and in thevicinity of the classifier dust-exhaust gas pipe and in the filter isset a vacuum of −25 to −45 mbar.
 19. Method according to claim 14,wherein exhaust gas from the calcining process is supplied at leastpartly in a bypass line directly to the filter and is conditioned withrespect to the volume flow and gas temperature independently of the gasflow through the roller grinding mill.
 20. Method according to claim 19,wherein the exhaust gas downstream of the filter and the plant fan is atleast partly returned to the mill.
 21. Method according to claim 15,wherein the pressure level in the roller grinding mill and extendinginto the filter is raised through the upstream mill fan.
 22. Methodaccording to claim 15, wherein exhaust gases of other milling systemsare supplied to the filter and dedusted.